2 Easter Eggs

Hidden Spicy Horse Logo

At the very end of Chapter 3, before you enter the Caterpillar's temple, you'll see two statues, one on the left, one on the right. Go behind the one on the left while shrunk and you'll see the Spicy Horse logo on it. This will win you a Trophy / Achievement.

Psychonauts Skeleton

In chapter four, the Executioner will chase you onto a slide, into a tunnel. On the right side, there will be a weak wall in the floor. Breaking it will reveal some stairs. You'll go down to see not only a memory, but a skeleton with a helmet that seems to a reference to the game Psychonauts.